Some prior art devices of this kind are e.g. known from NO 309619, NO 318487 and WO 03/029589. Other types of adjustable devices are found e.g. in U.S. Pat. No. 5,799,370 and FR 2368596.
Earlier height adjustment devices have been found to have a number of limitations in as regards the range of possible height adjustment which is present within the available area. Many of these known devices also have a tendency to tip over and out of a set position and thus lose their bearing capacity in the set position.